I agree, Intruder, that that is the most likely explanation, but the noise difference is so marked that the fan speed/shape must also be significantly different between the two. I wonder therefore if one characteristic of the Trent's three-shaft design is to produce a much slower rotating fan and therefore a lower pitched-noise? Hopefully an engineer will read this thread and comment. Incidentally I have yet to be close to an A340-600 at take-off but I would expect there to be a similarly marked difference between it and a CFM-56 powered -200 or -300.